This invention relates to electronic musical instruments, particularly those employing tunable circuits such as voltage-controlled oscillators and voltage-controlled filters.
Electronic musical instruments typically employ electronically tunable frequency-dependent circuits such as voltage-controlled oscillators (VCO's) and voltage-controlled filters (VCF's). The oscillators are used as musical tone signal sources, and the filters are used to process tone signals to achieve desired musical effects. The tuning is conventionally accomplished by applying an electronic control signal to select the operating frequency of the oscillator or the frequency characteristics of the filter. The tunable oscillator and filter circuits are commonly denominated "voltage-controlled". In most instances it is actually the control signal current which governs the tunable frequency characteristics, but ordinarily the current input to the tunable circuit is a function of the signal voltage, so the name is not entirely a misnomer even when current is the controlling parameter.
It is recognized in the electronic musical instrument art that pitch varies exponentially with position on the musical scale. For example, in terms of musical scale distance, middle C is halfway between the two notes an octave below and an octave above it; i.e. the scale is linear. But the frequency of middle C is not halfway between the frequencies of those other two notes; on the contrary, the frequency of middle C is twice that of the octave below it, while the upper octave frequency (twice the frequency of middle C) is four times the frequency of the octave below middle C. Thus, if the oscillator or filter tuning signal is linearly proportional to the musical scale position of a selected note, some means must be used to convert this linear relation into an exponential one.
The prior art has typically accomplished this (and incidentally accomplished the necessary voltage to current conversion as well) by using a conventional bipolar junction transistor as a signal conversion device. The transistor is first biased into the exponential region of its characteristic response curve, and then an input signal voltage linearly proportional to the musical scale position of the selected note is applied across its base and emitter electrodes. The resulting collector output current is an exponential function of the input voltage, and hence of the musical scale position of the selected note. Such an exponentially related current output is exactly what is needed to provide a tuning control signal for the so-called VCO's and VCF's discussed above. VCO and VCF circuits employing such control circuits are described in considerable detail in "Musical Engineer's Handbook", 1975 edition, Chapters 5 b and 5 d respectively. The foregoing work is available from the author and publisher, Bernie Hutchins, at 1 Pheasant Lane, Ithaca, New York 14850.
The problem with using bipolar transistors as exponential conversion devices in this fashion is that they drift with temperature and age. While various adjustment and compensation techniques are available, it is deemed preferable to attack the problem by providing an entirely different type of exponential conversion circuit, as the present invention does.